fix: fail fast on DeepL 403 and prevent silent untranslated PR creation

[Copilot]

Auto-translate silently produces PRs with English "translations" when the DeepL API key is invalid — every batch returns 403, the broad except Exception falls back to source text, and the workflow exits 0.

Changes

• Fail fast on 403/456: catch urllib.error.HTTPError explicitly; raise RuntimeError immediately on auth/quota errors instead of falling back to source text and burning through all remaining batches
• Surface DeepL's error body: reads the response payload (e.g. "Wrong endpoint. Please use the DeepL API Free endpoint…") and includes it in the error message
• Propagate failure: sync_locale returns {"error": ...} on abort (no file written); main exits non-zero when any locale fails → CI step fails → Check for modified files and PR-creation steps are skipped
• Strip API key: DEEPL_API_KEY.strip() guards against trailing whitespace/newlines in the secret value
• Summary table: shows ERROR row for failed locales instead of misleading coverage stats

── de-DE ──
  [de-DE] FATAL – DeepL auth/quota error (aborting remaining batches):
          HTTP Error 403: Forbidden – Wrong endpoint. Please use the DeepL API Free endpoint…
══════════════════════════════════════════════════════════════
de-DE          ERROR  (translation aborted – see above)

Repo owner action required: verify DEEPL_API_KEY in Settings → Secrets → Actions. Free-tier keys end with :fx (script auto-routes to api-free.deepl.com).